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Title: Environmental drivers of trait changes in Photorhabdus luminescens

item BLACKBURN, DANA - Brigham Young University
item CRAWFORD, BURKE - Brigham Young University
item Shapiro Ilan, David
item ADAMS, BYRON - Brigham Young University

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2015
Publication Date: 11/23/2016
Citation: Blackburn, D., Crawford, B., Shapiro-Ilan, D.I., Adams, B. 2016. Environmental drivers of trait changes in Photorhabdus luminescens. Biological Control. 92:145-152.

Interpretive Summary: Entomopathogenic nematodes, also known as beneficial nematodes, are environmentally friendly bio-insecticides that are used to control a wide variety of economically important insect pests. Beneficial nematodes kill insect pests with a help of their partner, a mutualistic symbiotic bacterium. These nematodes and their bacteria are routinely cultured in scientific laboratories for experimentation and mass-produced by companies for commercial applications. One problem with growing the nematodes is that some of their important properties, such as virulence (killing ability) and reproductive capacity, begin to deteriorate after multiple generations. The cause for nematode deterioration has been attributed largely to genetic causes. However, changes in the symbiotic bacteria when grown in artificial media have not been extensively studied. Therefore, the objective of this study was to determine if beneficial nematode bacteria change when cultured in different kinds of artificial media and if those changes are genetically based or if they stem from nutritional sources. Bacteria called Photorhabdus luminescens were grown in different artificial media. The bacteria did not change substantially in two of the three media tested, and in the third media, changes were recovered when re-grown in one of the other two media. Therefore, we conclude that changes in nematode symbiotic bacteria during in vitro culture are primarily nutritional. Also, we conclude that it is important to choose a nutritionally competent media for growing the bacteria to maximize growth and virulence for biological control purposes.

Technical Abstract: Biological control agents have become increasingly important in integrated pest management programs. However, certain traits of these agents that are needed for efficient biocontrol often decrease or are lost during in vitro rearing. Trait deterioration can result from genetic or environmental causes (such as nutrition). Entomopathogenic nematodes (EPNs) are biocontrol agents that kill their insect targets with the help of a symbiotic bacterium. EPNs and their bacterial symbionts often exhibit trait deterioration when reared under laboratory conditions. EPN trait deterioration has been attributed (at least in part) to genetic causes; however, the underlying causes of trait deterioration in the bacterial endosymbiont have not been explored. In this study the EPN symbiont Photorhabdus luminescens was monitored for the deterioration of three traits; inclusion body production, reproductive potential, and virulence, in three different nutritional environments; lipid liquid medium (LLM), nutrient broth (NB), and tryptic soy broth+yeast extract (TSY). Significant trait deterioration did not occur for any of the traits in any environment. However, there was an increase in inclusion body production in TSY. Additionally, there were differences in growth rates within NB and TSY sub-cultured population lines and one TSY sub-population line was less virulent than the other two. However, returning bacteria to LLM restored all traits to wild-type levels. We infer the observed trait deterioration in Photorhabdus was minimal, and the deterioration that was observed appeared to be driven by environmental conditions as opposed to stable genetic changes. Our data suggest that variation among important biological control traits of in vitro cultures of Photorhabdus luminescens is more likely due to environmental variation than inadvertent laboratory selection or other genetic processes.